In order to boot from cd or dvd you need to change the guest virtualization type from HVM (fully virtualized) to PV (paravirtualized).
$ sudo xenstore-read vm | xargs -n1 basename
54a95df1-d4f3-67f7-46d3-6dcd1d1143ab
xe vm-param-set HVM-boot-p olicy="BIOS order" uuid=[uuid of your vm]
After you have booted from dvd, change back to fully virtualized mode:
xe vm-param-set HVM-boot-p olicy="" uuid=[uuid of your vm]
Read more about difference between HVM and PV:
http://serverfault.com/questions/222010/difference-between-xen-pv-xen-kvm-and-hvm